Megaflo 28 system boiler

[slippyr4]

Constant current fed into the thermistor, voltage developed varies inversely with resistance.

That would be a ridiculous design for an application such as this. A whole IC for a constant current source, vs a SMT resistor which would cost well under a penny. Both product an output of a voltage that then needs to be read with an ADC pin on the microcontroller.

To you maybe it does seem like rubbish, but then do you have skills and experience in electronics design ?

Yes, a career in embedded microelectronics.

I respect your knowlege of RTD's - i've never used them, and I guess that's where your talk of elements comes from.

 

[bernardgreen]

That would be a ridiculous design for an application such as this. A whole IC for a constant current source, vs a SMT resistor which would cost well under a penny. Both product an output of a voltage that then needs to be read with an ADC pin on the microcontroller.

Using a discrete IC for the current source would not be cost effective but since there is a micro-controller on the PCB which has probaby been custom designed fro the application then that micro-controller could have the constant current source as one of its integrated peripherals.

 

[slippyr4]

it wouldn't be cost effective to use an ASIC for a boiler which would sell only a few thousand units in it's lifetime.

 

[bernardgreen]

I was thinking along the lines of a Microchip PIC The 16F1777 has 2 or 3 Op amps one of which could form the constant current source.

 

[AGAS]

Faulty thermistor, but you got good readings from it. Strange to be faulty and give good readings at the same time.

thermistors can break down at high temps when measuring good at lower temps

 

[Dan Robinson]

Seems to be the most common form of failure in my recent experience.

Armchair googling continues at its finest. Let's hope everything is spell checked first

 

[bernardgreen]

Let's how everything is spell checked first

?

 

[Dan Robinson]

Bloody autocorreckt... Oh the irony.

 

[Agile]

Some NTCs used in boilers will go open circuit above a certain temperature. That is the thermal effect of a faulty connection internally to the element.

A far simpler temperature monitoring circuit will be to just use a series resistor of several times the max value of the thermistor. That will be largely a constant current source and only use a very cheap resistor. Then the voltage across the thermistor is measured and inversely proportional to the temperature with the NTC types as used in boilers.

 

[darkart]

Just like to say thanks for all your advice; purchased a new NTC sensor, fitted it and all is now ok